Replace ateredis with atepg

[Jet Chiang (supreme-gg-gg)]

Summary

A follow up to #640 where we introduced PostgreSQL as an alternative storage backend, selected conditionally in ateapi.

• Deleted ateredis, its tests, and its dependencies
• Removed Redis backend selection and configuration so ateapi always connects to Postgres
• Replaced Valkey resources with Postgres in the standard and Kind deployment paths and simplified install script
• Replaced miniredis fixtures with isolated Postgres testcontainers and added centralized helpers for seeding resources
• Renamed Redis-specific debug flush command to backend-neutral debug-clear-store in CLI
• Updated comments and docs where applicable

Benchmarking

Extensive benchmarking have been performed to evaluate Redis vs Postgres, and results can be found in these two documents:

• https://docs.google.com/document/d/10K0wB6aTeFkJCL4HN3NbLJCdFGoLYdhIcqkFnqFHkKc/edit?usp=sharing
• https://docs.google.com/document/d/12-ko_BFHcBo_nJkx9f4B7zMbiiWKC2saGhMhZG3aQ-s/edit?usp=sharing

[Eitan Yarmush (EItanya)]

Very large PR, but so far this is my main comment